Targeting of influenza epitopes to murine CR1/CR2 using single-chain antibodies

DNA and protein-generated chimeric molecules for delivery of influenza viral epitopes in mouse and humanized NSG transfer models

Purified subunit viral antigens are weakly immunogenic and stimulate only the antibody but not the T cell-mediated immune response. An alternative approach to inducing protective immunity with small viral peptides may be the targeting of viral epitopes to immunocompetent cells by DNA and protein-engineered vaccines. This review will focus on DNA and protein-generated chimeric molecules carrying engineered fragments specific for activating cell surface co-receptors for inducing protective antiviral immunity. Adjuvanted protein-based vaccine or DNA constructs encoding simultaneously T- and B-cell peptide epitopes from influenza viral hemagglutinin, and scFvs specific for costimulatory immune cell receptors may induce a significant increase of anti-influenza antibody levels and strong CTL activity against virus-infected cells in a manner that mimics the natural infection. Here we summarize the development of several DNA and protein chimeric constructs carrying influenza virus HA317-41 fragment. The generated engineered molecules were used for immunization in intact murine and experimentally humanized NSG mouse models.

Targeting of Influenza Viral Epitopes to Antigen-Presenting Cells by Genetically Engineered Chimeric Molecules in a Humanized NOD SCID Gamma Transfer Model

Antiviral DNA vaccines are a novel strategy in the vaccine development field, which basically consists of the administration of expression vectors coding viral antigen sequences into the host's cells. Targeting of conserved viral epitopes by antibody fragments specific to activating cell surface co-receptor molecules on antigen-presenting cells could be an alternative approach for inducing protective immunity. It has been shown that FcγRI on human monocytes enhances antigen presentation in vivo. Various DNA constructs, encoding a Single-chain variable antibodies (scFv) from mouse anti-human FcγRI monoclonal antibody, coupled to a sequence encoding a T- and B-cell epitope-containing influenza A virus hemagglutinin inter-subunit peptide were inserted into the eukaryotic expression vector system pTriEx-3 Neo. The constructed chimeric DNA molecules were expressed by transfected Chinese hamster ovary cells and the ability of the engineered proteins to interact with FcγRI-expressing cells was confirmed by flow cytometry. The fusion protein induced a strong signal transduction on human monocytes via FcγRI. The expression vector pTriEx-3 Neo containing the described construct was used as a naked DNA vaccine and introduced directly to experimental humanized NOD SCID gamma mice with or without boosting with the expressed fusion protein. Immunization with the generated DNA chimeric molecules and prime-boost with the expressed recombinant proteins induced significant serum levels of anti-influenza immunoglobulin G antibodies and strong cytotoxic T lymphocyte activity against influenza virus-infected cells in humanized animals.

The versatile functions of complement C3-derived ligands

The complement system is a major component of immune defense. Activation of the complement cascade by foreign substances and altered self-structures may lead to the elimination of the activating agent, and during the enzymatic cascade, several biologically active fragments are generated. Most immune regulatory effects of complement are mediated by the activation products of C3, the central component. The indispensable role of C3 in opsonic phagocytosis as well as in the regulation of humoral immune response is known for long, while the involvement of complement in T-cell biology have been revealed in the past few years. In this review, we discuss the immune modulatory functions of C3-derived fragments focusing on their role in processes which have not been summarized so far. The importance of locally synthesized complement will receive special emphasis, as several immunological processes take place in tissues, where hepatocyte-derived complement components might not be available at high concentrations. We also aim to call the attention to important differences between human and mouse systems regarding C3-mediated processes.

A new mouse anti-mouse complement receptor type 2 and 1 (CR2/CR1) monoclonal antibody as a tool to study receptor involvement in chronic models of immune responses and disease

Although reagents are available to block mouse complement receptor type 2 and/or type 1 (CR2/CR1, CD21/CD35) function in acute or short term models of human disease, a mouse anti-rat antibody response limits their use in chronic models. We have addressed this problem by generating in Cr2−/− mice a mouse monoclonal antibody (mAb 4B2) to mouse CR2/CR1. The binding of murine mAb 4B2 to CR2/CR1 directly blocked C3dg (C3d) ligand binding. In vivo injection of mAb 4B2 induced substantial down regulation of CR2 and CR1 from the B cell surface, an effect that lasted six weeks after a single injection of 2 mg of mAb. The 4B2 mAb was studied in vivo for the capability to affect immunological responses to model antigens. Pre-injection of mAb 4B2 before immunization of C57BL/6 mice reduced the IgG1 antibody response to the T-dependent antigen sheep red blood cells (SRBC) to a level comparable to that found in Cr2−/− mice. We also used the collagen-induced arthritis (CIA) model, a CR2/CR1-dependent autoimmune disease model, and found that mice pre-injected with mAb 4B2 demonstrated substantially reduced levels of pathogenic IgG2a antibodies to both the bovine type II collagen (CII) used to induce arthritis and to endogenous mouse CII. Consistent with this result, mice pre-injected with mAb 4B2 demonstrated only very mild arthritis. This reduction in disease, together with published data in CII-immunized Cr2−/− mice, confirm both that the arthritis development depends on CR2/CR1 receptors and that mAb 4B2 can be used to induce biologically relevant receptor blockade. Thus mAb 4B2 is an excellent candidate for use in chronic murine models to determine how receptor blockage at different points modifies disease activity and autoantibody responses.

Generation of gene-engineered chimeric DNA molecules for specific therapy of autoimmune diseases

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the development of self-reactive B and T cells and autoantibody production. In particular, double-stranded DNA-specific B cells play an important role in lupus progression, and their selective elimination is a reasonable approach for effective therapy of SLE. DNA-based vaccines aim at the induction of immune response against the vector-encoded antigen. Here, we are exploring, as a new DNA-based therapy of SLE, a chimeric DNA molecule encoding a DNA-mimotope peptide, and the Fv but not the immunogenic Fc fragment of an FcγRIIb-specific monoclonal antibody. This DNA construct was inserted in the expression vector pNut and used as a naked DNA vaccine in a mouse model of lupus. The chimeric DNA molecule can be expressed in eukaryotic cells and cross-links cell surface receptors on DNA-specific B cells, delivering an inhibitory intracellular signal. Intramuscular administration of the recombinant DNA molecule to lupus-prone MRL/lpr mice prevented increase in IgG anti-DNA antibodies and was associated with a low degree of proteinuria, modulation of cytokine profile, and suppression of lupus nephritis.

Enhancing DNA immunization by targeting ASFV antigens to SLA-II bearing cells

One of the main criticisms to DNA vaccines is the poor immunogenicity that they confer on occasions, at least in large animals. Confirming this theory, immunization with plasmid DNA encoding two African swine fever virus genes in frame (pCMV-PQ), failed in inducing detectable immune responses in pigs, while it was successful in mice. Aiming to improve the immune responses induced in swine, a new plasmid was constructed, encoding the viral genes fused in frame with a single chain variable fragment of an antibody specific for a swine leukocyte antigen II (pCMV-APCH1PQ). Our results clearly demonstrate that targeting antigens to antigen professional cells exponentially enhanced the immune response induced in pigs, albeit that the DNA vaccine was not able to confer protection against lethal viral challenge. Indeed, a viremia exacerbation was observed in each of the pigs that received the pCMV-APCH1PQ plasmid, this correlating with the presence of non-neutralizing antibodies and antigen-specific SLA II-restricted T-cells. The implications of our discoveries for the development of future vaccines against African swine fever virus and other swine pathogens are discussed.

Marine gastropod hemocyanins as adjuvants of non-conjugated bacterial and viral proteins

Killed viral vaccines and bacterial toxoids are weakly immunogenic. Numerous compounds are under evaluation as immunological adjuvants and peptide-carriers to improve the immune response. The hemocyanins, giant extracellular copper proteins in the blood of many mollusks, are widely used as immune stimulants. In the present study we investigated the adjuvant properties of hemocyanins isolated from marine gastropods Rapana thomasiana and Megathura crenulata. An immunization with Influenza vaccine or tetanus toxoid combined with Rapana thomasiana hemocyanin (RtH) and Keyhole limpet hemocyanin (KLH) in mice induced an anti-influenza cytotoxic response lasting at least 5 months and an antibody response to viral proteins. The IgG antibody response to the tetanus toxoid (TT) combined with RtH or KLH was comparable to the response of the toxoid in complete Freund's adjuvant. The results obtained demonstrate that the both hemocyanins are acceptable as potential bio-adjuvants for subunit vaccines.

CD16/32-specific biotinylated 2.4G2 single-chain Fv complexed with avidin-FITC enhances FITC-specific humoral immune response in vivo in a CD16-dependent manner

Fcgamma receptors (FcgammaRs) play an essential role in the regulation of immune response due to their ability to bind immune complexes. Activating FcgammaRs may facilitate antigen presentation and dendritic-cell maturation, while in the late phase of the immune response, the inhibitory FcgammaRIIb may down-regulate B-cell activation upon cross-linking with activating receptors. In this study, we investigated the in vivo role of FcgammaRs on the modulation of humoral immune response. In order to get well-defined immune complexes that can bind to both the activating and the inhibitory FcgammaRs, we designed a mono-biotinylated single-chain fragment variable construct from the rat anti-mouse CD16/32 clone 2.4G2, linked to avidin-FITC, and tested its effect on the FITC-hapten-specific T-independent type 2 (TI-2) and T-dependent (TD) immune response. When injected intravenously in mice, the complex bound to a small portion of B220+, CD11b(high) and CD11c(high) cells and was localized in the spleen on marginal zone macrophages 15 min after treatment. When applied as a booster following primary immunization with TI-2 (FITC-dextran) or TD (FITC-keyhole limpet haemocyanin) antigens, the complex elevated the number of hapten-specific IgM/IgG-producing B cells. This effect was diminished in CD16KO mice, suggesting that the activating-type FcgammaRIII might be a key mediator of this mechanism.

Proteome-guided search for influenza A B-cell epitopes

The influenza A linear peptide epitopes recognized by murine antibodies, and currently cataloged at http://www.immuneepitope.org, were examined for the identity score to the host mouse proteome. It was found that almost all of the linear viral determinants are (or contain) regions formed by pentapeptide fragments with no or only very low similarity to the murine proteins. The present study adds to previous reports in suggesting a main role of amino acid sequence similarity in the modulation and definition of the B-cell epitope repertoire, inspiring innovative vaccine approaches able to avoid cross-reactive autoimmune collateral phenomena, and addressing future research in the study of immunity against the influenza A virus and infectious diseases in general.

Expression of a functional single-chain variable-fragment antibody against complement receptor 1 in Streptococcus gordonii

Streptococcus gordonii, an oral commensal organism, is a candidate vector for oral-vaccine development. Previous studies have shown that recombinant S. gordonii expressing heterologous antigens was weakly immunogenic when delivered intranasally. In this study, antigen was specifically targeted to antigen-presenting cells (APC) in order to potentiate antigen-APC interactions and increase the humoral immune response to the antigen. To achieve this goal, a single-chain variable-fragment (scFv) antibody against complement receptor 1 (CR1) was constructed. Anti-CR1 scFv purified from Escherichia coli was able to bind to mouse mixed lymphocytes and bone marrow-derived dendritic cells. The in vivo function of the anti-CR1 scFv protein was assessed by immunizing mice intranasally with soluble scFv and determining the immune response against the hemagglutinin (HA) peptide located on the carboxy terminus of the scFv. The serum anti-HA immunoglobulin G (IgG) immune response was dose dependent; as little as 100 ng of anti-CR1 scFv induced a significant IgG immune response, while such a response was minimal when the animals were given an unrelated scFv. The anti-CR1 scFv was expressed in S. gordonii as a secreted protein, which was functional, as it bound to dendritic cells. Mice orally colonized by the anti-CR1-secreting S. gordonii produced an anti-HA IgG immune response, indicating that such an approach can be used to increase the immune response to antigens produced by this bacterium.

Novel roles for murine complement receptors type 1 and 2 I. Regulation of B cell survival and proliferation by CR1/2

Innate components of the immune system, such as complement are known to have a modulatory effect on adaptive immune responses. Complement receptors are expressed by both B and T lymphocytes and play part in antigen presentation and cellular activation and adhesion events. On murine B cells type 1 and 2 complement receptors (CR1/2) are expressed and form a co-receptor complex together with CD19 and CD81. We used CR1/2 specific antibodies to assess the role these receptors might play in regulating cell cycling events of B cells. We show that a CR1/2 specific antibody fragment, 7G6 scFv can induce the proliferation of mature B cells. This effect is countermodulated by FcR crosslinkage and enhanced by BCR engagement. The proliferative effect is severely impaired in Cr2-/- animals, strengthening the involvement of CR1/2. Transitional B cells are prone to apoptotic death by selection events, yet they are rescued from apoptosis by CR1/2 crosslinkage. CR1/2 ligation by 7G6 scFv alone can induce nuclear translocation of NF-kappaB, supporting the above observations. We conclude that engagement of complement receptor 2 of B cells promotes the survival of both mature and transitional B cells. This activity supplements the previously described adjuvant effects of complement.

Low doses of antigen coupled to anti-CR2 mAbs induce rapid and enduring IgG immune responses in mice and in cynomolgus monkeys

The complement system and B cell complement receptor 2 (CR2), specific for C component C3dg, play important roles in both the innate and adaptive immune response. We used hapten and protein conjugates of anti-CR2 mAbs as models for C3dg-opsonized antigens and immune complexes to examine the handling of and immune response to these reagents in mice and in non-human primates (NHP). Mice immunized and boosted i.v. with only 100 ng of Alexa 488 rat anti-mouse CR1/2 mAb 7G6 had strong IgG immune responses to the Alexa 488 hapten and to rat IgG, compared to very weak immune responses in mice treated with a comparable isotype control; larger doses of Alexa 488 mAb 7G6 did not increase the immune response. A vaccine constructed by cross-linking anthrax protective antigen to mAb 7G6 proved to be effective at low doses in generating sufficiently high titer serum IgG antibodies to neutralize anthrax lethal toxin in vitro and to protect mice from i.v. challenge with anthrax lethal toxin. When biotinylated HB135, a mouse mAb specific for human CR2, was injected i.v. into NHP, the probe manifested the same initial marginal zone B cell binding and subsequent localization to follicular dendritic cells as we have previously reported for comparable experiments in mice. Moreover, i.v. immunization of NHP with 1 microg/kg of Alexa 488 mAb HB135 promoted an IgG immune response to the Alexa 488 hapten and to mouse IgG. Taken together, these results demonstrate the efficacy of using anti-CR2 mAbs as antigen carriers for i.v. immunization with small amounts of antigens without adjuvant.

Immunization with a DNA chimeric molecule encoding a hemagglutinin peptide and a scFv CD21-specific antibody fragment induces long-lasting IgM and CTL responses to influenza virus

Killed viral vaccines are known to induce primarily antibody responses. By contrast DNA vaccination using naked DNA encoding viral antigens induces both humoral and cellular immune responses. Various approaches have been used to construct DNA vaccines with build-in adjuvanticity. We hypothesized that sequences encoding a common epitope of influenza A virus hemagglutinin jointed to sequences encoding a single-chain variable fragment (scFv) antibody fragment to a costimulatory B cell surface receptor would result in the in vivo expression of a chimeric viral peptide with increased immunogenicity. Such a hybrid DNA molecule was constructed by us, encoding a T and B cell epitope-containing influenza hemagglutinin peptide and a scFv antibody fragment binding to mouse complement receptors I and II (CR1 and CR2). A single immunization with a plasmid containing the described construct induced a strong anti-influenza cytotoxic response lasting for more than six months and a weak antibody response.

Antigen receptor-mediated signaling pathways in transitional immature B cells

Engagement of antigen receptors on immature B cells induces apoptosis, while at the mature stage, it stimulates cell activation and proliferation. The difference in B cell receptor (BCR)-mediated signaling pathways regulating death or survival of B cells is not fully understood. We aimed to characterize the pathway leading to BCR-driven apoptosis. Transitional immature B cells were obtained from the spleen of sublethally irradiated and auto-reconstituted mice. We have detected a short-lived BCR-driven activation of mitogen-activated protein kinases (ERK1/2 and p38 MAPK) and Akt/PKB in transitional immature B cells that correlated with the lack of c-Fos expression, reduced phosphorylation of Akt substrates and a susceptibility for apoptosis. Simultaneous signaling through BCR and CD40 protected immature B cells from apoptosis, however, without inducing Bcl-2 expression. The BCR-induced apoptosis of immature B cells is a result of the collapse of mitochondrial membrane potential and the subsequent activation of caspase-3.

Targeting with scFv: immune modulation by complement receptor specific constructs

The fate of a microbe in the host is determined by various molecules of the innate immune system, which recognize the microbe and enhance its interaction with antigen presenting cells. This 'natural targeting' phenomenon, however, does not function when antigens with limited immunogenicity enter the host. Peptide vaccines, for instance, require adjuvants to induce immune responses. As a surrogate for the natural targeting mechanisms, antibodies against selected receptors of antigen presenting cells, conjugated with the peptides, could be used as targeting devices. Here we review various antibody-mediated antigen-targeting strategies, paying special attention to complement receptor-mediated targeting. We also describe and summarize our method of single-chain antibody-mediated targeting of viral epitopes to complement receptor type two and discuss the perspectives of single-chain antibody-mediated antigen targeting.

Mucosal type mast cells express complement receptor type 2 (CD21)

Fragments of complement component C3 generated upon activation of the cascade play an important role in the induction and regulation of immune responses. Receptors interacting with various fragments of this versatile complement protein are expressed on a wide variety of cell types, including lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes and consequently, C3-products may influence several biological functions at different sites of the body, where complement activation occurs. Regarding the expression of various C3-receptors on mast cells, mainly rodent serosal type mastocytes have been investigated so far. It has been known for a long time that C3a triggers the release of mediators of immediate type hypersensitivity via binding to serosal-type cells. Complement receptor type 1 (CR1/CD35) and type 2 (CR2/CD21) interacting with the larger activation products, such as C3b and C3d, have so far been shown on serosal type mast cells only. In this study, the expression of CR1/2 on mucosal type mast cells is demonstrated. Using mouse CR1/2 specific single chain antibodies and the natural ligand C3d in cytofluorimetric measurements, we show that the rat mucosal mast cell line RBL-2H3 and mouse bone marrow-derived mast cells (BMMC) express CD21. RT-PCR experiments carried out with mouse CR1 and CR2 specific primers show CD21, but not CD35 specific products in BMMC. It is also demonstrated that, in contrast to serosal type mast cells, mucosal mastocytes do not express CD19. In an attempt to reveal the possible function of CR2 on mucosal type mast cells, the effect of receptor-clustering was tested regarding degranulation, Ca-response and IL-6 production, but no CR2-mediated change was detected in any of these processes.

Bone marrow-derived mast cell differentiation is strongly reduced in histidine decarboxylase knockout, histamine-free mice

Mast cells are differentiated in vitro from bone marrow precursors. In this study the development of bone marrow-derived mast cells was examined from histidine decarboxylase deficient (HDC-/-) and wild-type mice in the presence of IL-3. The number of non-adherent, tryptase- and c-kit-positive mast cells in bone marrow-derived cultures of HDC(-/-) mice was decreased compared to that of wild-type (HDC+/+) animals, but within the tryptase- and c-kit-positive cells there was no difference in the expression intensity of both markers between the two groups. Furthermore, less serine proteases mMCP5, mMCP6 and FcepsilonRIalpha mRNA were detected in bone marrow-derived cell cultures originating from HDC-/- mice. Antigen-provoked degranulation through high-affinity FcepsilonI receptor was also lower in HDC-/- mice. The colony assays in semisolid medium yielded a significantly lower ratio of mixed colonies and higher proportion of macrophage colonies from HDC-/- mice-derived bone marrow compared to the wild-type. In the course of the differentiation of HDC-/- --derived mast cells exogenously added histamine is unable to substitute the endogenously missing histamine. Concordantly, alpha-fluoromethyl-histamine, the specific inhibitor of HDC, revealed only a marginal inhibition on the differentiation of tryptase-positive mast cells from wild-type mice. These findings suggest that the effect of histamine on the IL-3-dependent development of bone marrow-derived mast cell differentiation during the early period is crucial and irreplaceable.

Modeling the presentation of C3d-coated antigen by B lymphocytes: enhancement by CR1/2-BCR co-ligation is selective for the co-ligating antigen

We have used a set of single-chain variable fragment antibodies (sc) genetically fused with an influenza hemagglutinin-derived peptide as a means to investigate the role of CR1 and CR2 in antigen presentation by B cells. When incubated with the B cell lymphoma 2PK3, peptide-containing sc specific for either CR1 or CR1/2 mediated activation of the hemagglutinin peptide-specific T cell line IP-12-7, as assessed by IL-2 production. Efficient presentation was dependent on the binding of the constructs to CR1/2, implying that receptor-mediated endocytosis is responsible for the effect. Cross-linkage of CR1/2 or CD19 by mAb did not increase the extent of T cell activation. However, when CR1/2 was co-ligated with the BCR--using either polyclonal goat anti-mouse IgG or recombinant protein LA--the antigen concentration required to activate T cells decreased by two orders of magnitude. Moreover, this enhancement was selective for the antigen included in these complexes and did not affect the presentation of a free peptide or of antigen bound to CR1/2 excluded from the complexes. These results suggest that B cells may bind various C3d-coated antigens at a time, but only the one which reacts with the BCR will be processed with high efficiency. This mechanism may ensure the specificity of cognate T cell help.

Immunomodulatory functions of murine CR1/2

C3-fragments generated upon complement activation play an important role in the formation and regulation of immune responses. Receptors interacting with various activation fragments of this versatile complement component are expressed on a wide variety of cell types, such as lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes, consequently C3-products may influence several biological functions at different sites of the body, where complement activation takes place. In the last decade, genes, protein structure and functions played by murine complement receptors CR1 and CR2 (mCR1/2) have been deciphered. In this review, we wish to relate these properties, and fit it into the context of events following in vivo complement activation. We separately address the roles played by murine mCR1/2 as BCR coreceptor and as BCR independent structure, and propose a mchanism for the utilization of antigen-C3d conjugates bound on B cells. Finally, we raise some of the questions that remain to be elucidated in order to get a more precise picture of the functions of mCR1/2.